A Full-Field Wideband Modal Testing Method for Turbine Blades

In order to accurately measure the low- and high-order modes of turbine blades, this study proposes a method that integrates piezoelectric ceramic excitation with time-averaged electronic speckle pattern interferometry (TA-ESPI). The piezoelectric exciter provides wideband and high-output excitation, effectively stimulating both low- and high-order blade modes. The TA-ESPI technique captures the vibration signals with high displacement sensitivity, enabling full-field mode shape measurement without the need for high-speed cameras. Additionally, an equivalent strain principle based on deflection curvature is introduced to extract strain modes from displacement modes for identifying potential fatigue failure areas (PFFAs). Experimental validation on an aero-engine turbine blade successfully identified the first eight natural frequencies (up to 7753 Hz), a significant advancement over the conventional method which identified only two. The measured first-order frequency showed a high agreement (deviation < 2%) with the impact hammer test, confirming accuracy. The extracted strain modes for the first six orders clearly revealed the PFFAs, which aligned well with actual failure regions. The proposed method proves to be an effective, practical, and efficient modal testing technique for turbine blades, offering a substantial improvement in bandwidth over established methods.